1. Field of the Invention
The present invention relates to a motor armature and a motor.
2. Description of the Related Art
As causes of inefficiency in driving motors, there have been known copper losses generated by a resistance component of a winding, core losses generated by physical properties of a core constituting an armature, and mechanical losses generated by friction in rotational motion. While the core losses can be further classified into hysteresis losses and eddy current losses, the eddy current losses account for a large proportion. Therefore, it is necessary to reduce the eddy current losses in order to improve motor efficiency by reducing the core losses. Eddy currents are currents generated in the core by an electromagnetic induction action. When a magnetic flux in the core changes, the eddy currents flow in a direction to oppose the change in direction of the magnetic flux. The eddy current losses are energy lost as Joule heat by the flow of the eddy currents.
A motor core is normally formed by stacking a plurality of electromagnetic steel sheets. Each of the electromagnetic steel sheets is defined by a planar body where an insulating layer is formed on a surface. An eddy current loss Pe in a planar conductor is known to be expressed by the following expression 1 using a thickness h of the conductor, a frequency f, a maximum magnetic flux density Bm, and a resistivity ρ of a magnetic material. According to the expression, it is understood that the eddy current loss becomes ¼ by decreasing a thickness of the electromagnetic steel sheet used for the core to ½. That is, it is understood that the eddy current loss is reduced by decreasing the thickness of the electromagnetic steel sheet.
                    Pe        =                                            π              2                                      6              ⁢              ρ                                ⁢                      Bm            2                    ⁢                      f            2                    ⁢                      h            2                                              Expression        ⁢                                  ⁢        1            
Also, a method for reducing the core losses by devising a shape of a stator core has been conventionally proposed (See, for example, Japanese Patent Laid-Open Nos. 2011-160578 and 2012-135123).
Japanese Patent Laid-Open No. 2011-160578 discloses a core in which a plurality of slits parallel to a compressive stress are provided in a back yoke portion. It is described that the compressive stress by shrink fitting is relaxed, and the core losses can be reduced by employing the configuration.
However, when the thickness of the electromagnetic steel sheet is decreased in order to reduce the eddy current losses, a mechanical strength of the electromagnetic steel sheet is disadvantageously lowered, and it disadvantageously becomes difficult to machine the electromagnetic steel sheet.
When the slits are provided in the back yoke portion in order to reduce the eddy current losses, the mechanical strength of the electromagnetic steel sheet is also disadvantageously lowered.